1. Field of the Invention
This invention relates to polymer electrolyte membrane fuel cells suitable for direct oxidation of liquid fuels. More particularly, this invention relates to direct methanol fuel cells which eliminate the need for pumps, blowers and humidifiers in order to operate.
2. Description of Related Art
Direct methanol polymer electrolyte membrane fuel cells are prime candidates for both vehicular and stationary uses due to their inherent simplicity (no external reformers) and potential high energy densities (liquid fuels). In addition, direct methanol polymer electrolyte membrane fuel cells have the potential for replacing rechargeable batteries due to the possibility of a zero recharge time. However, the current state of the art in direct methanol polymer electrolyte membrane fuel cells requires external means, such as pumps and blowers for introducing reactants into and removing reaction products from the fuel cell. For example, U.S. Pat. No. 5,573,866 to Van Dine et al. teaches a polymer electrolyte membrane fuel cell which directly oxidizes liquid methanol fuel that is fed into the anode chamber from a liquid methanol storage container. The liquid methanol is mixed with water in the anode chamber. Some of the methanol and water cross over the membrane into the cathode chamber and into a process air stream. The methanol and water are removed from the cathode chamber by evaporation into the process air stream, which is then directed into a condenser/radiator. The methanol and water vapors are condensed in the condenser/radiator, from whence the condensed water and methanol are returned to the anode chamber of the cell. The evaporating cathode process air stream, which is provided to the cathode chamber by means of a fan, provides oxygen for the fuel cell reaction, and also cools the cell. See also U.S. Pat. No. 4,390,630 to Kawana et al. which also teaches a methanol fuel cell. It will be appreciated that, to the extent direct methanol fuel cells require the use of external devices for operation, such as fans, blowers and humidifiers, they are not suitable for use in vehicular applications or as a replacement for rechargeable batteries.
Accordingly, it is one object of this invention to provide a polymer electrolyte membrane fuel cell suitable for generating electricity by direct oxidation of methanol that does not require the use of external means for providing oxidant to the fuel cell and/or removing water from the fuel cell. More particularly, it is one object of this invention to provide an electricity-producing, self-sustaining direct methanol polymer electrolyte membrane fuel cell.
It is another object of this invention to provide a method for generating electricity by direct oxidation of methanol that eliminates the need for auxiliary equipment such as fans, blowers and/or humidifiers for delivery to the fuel cell and removal from the fuel cell of reactants and reaction products, respectively.
These and other objects of this invention are addressed by an apparatus for generating electricity comprising an enclosure suitable for containment of a liquid hydrocarbon fuel and at least one air-breathing polymer electrolyte membrane fuel cell unit disposed in a wall of the enclosure with an anode side facing an interior of the enclosure. The at least one air-breathing polymer electrolyte membrane fuel cell unit comprises a polymer electrolyte membrane having an anode side and a cathode side, an anode electrode disposed on the anode side of the polymer electrolyte membrane and a cathode electrode disposed on the cathode side of the polymer electrolyte membrane. A porous metal cathode current collector having a centrally disposed active region is disposed on the cathode side of the polymer electrolyte membrane and a porous metal anode current collector is disposed on the anode side of the polymer electrolyte membrane. In accordance with a particularly preferred embodiment of this invention, the porous metal cathode current collector and the porous anode current collector are made from a foam metal. As will be shown, critical to the operation of the polymer electrolyte membrane fuel cell of this invention without the need for auxiliary pumps, fans, humidifiers, etc. are the porosity and pore sizes of the porous current collectors.
In accordance with one embodiment of the method of this invention, methanol is supplied through a porous anode current collector of a polymer electrolyte membrane fuel cell to the anode electrode and oxidant is supplied at substantially atmospheric pressure by means of diffusion through a porous cathode current collector to the cathode electrode of the polymer electrode membrane fuel cell. The methanol is directly oxidized in the fuel cell, resulting in the generation of water, carbon dioxide and electricity. The water generated in the fuel cell diffuses out of the fuel cell through the porous cathode current collector. In operation, there exists substantially no pressure differential between the anode side and the cathode side of the polymer electrolyte membrane fuel cell.